The present invention relates to interferometers, and particularly to a phase reading all fiber optic interferometer which includes a method and apparatus for measuring phase difference signals from the interferometer over an extended dynamic range of operation.
Interferometers typically comprise devices which provide for the propagation of two interfering light waves, with the phase difference between the light waves being dependent upon the difference in the respective optical path lengths traveled by the two light waves within the interferometer. The phase difference between the two interfering waves can also be influenced by external forces such as rotation of the interferometer. Thus, interferometers generally provide an output signal whose intensity is dependent upon the phase difference between the waves. Various methods and devices for detecting and measuring this phase difference have been devised, but each has been shown to have problems or limitations under certain operating conditions.
Devices for measuring the phase difference have often found application in interferometers which are used for rotation sensing. Thus, although the method and apparatus disclosed herein for detecting and measuring the phase difference output signal is useable with all conventional interferometers, its configuration and operation may best be described in connection with fiber optic rotation sensors, which comprise one preferred embodiment of the invention.
Fiber optic rotation sensors typically comprise a loop of fiber optic material to which light waves are coupled for propagation around the loop in opposite directions. Rotation of the loop creates a relative phase difference between counter-propagating waves, in accordance with the well known "Sagnac effect", with the amount of phase difference corresponding to the velocity of rotation. The counter-propagating waves, when recombined, interfere constructively or destructively to produce an optical output signal which varies in intensity in accordance with the rotation rate of the loop. Rotation sensing is commonly accomplished by detection of this optical output signal.
Various techniques have been devised to increase the sensitivity of fiber optic rotation sensors to small rotation velocities. For example, one open-loop technique involves phase modulating the counter-propagating light waves at a first harmonic frequency. The rotation rate may then be determined by phase sensitive detection of a component in the optical output signal at the phase modulation frequency. The amplitude of this component is proportional to the rotation rate. However, this technique is not available for detecting large rotation rates because the optical output signal defines a waveform which repeats itself periodically as the rotation rate increases or decreases. Thus, the amplitude of the measured component is the same at each periodic repetition of the output signal, even though the associated loops rotation rate is different. In addition, the sensitivity of the device becomes essentially zero at some locations on the repeating signal waveform, causing a nonlinear response of the device. Such techniques are difficult to use in many applications requiring rotation sensing over an extended dynamic range.
Another technique which involves an open-loop configuration involves a single sideband detection scheme such as the one described in D. Eberhard and E. Voges, "Fiber Gyroscope with Phase-Modulated Single-Sideband Detection," Opt. Lett. 9, 22 (1984). However, this approach is not feasible since it requires a wide band phase modulator which is not presently available in fiber-optic form.
Still another approach to rotation sensing involves a signal processing technique, as described in K. Bohm, P. Marten, E. Weidel, and K. Peterman, "Direct Rotation-Rate Detection With A Fiber-Optic Gyro By Using Digital Data Processing," Electron. Lett. 19, 997 (1983). In this approach, like the technique described above, the counter-propagating waves are phase modulated at a selected frequency. An odd harmonic and an even harmonic of the output signal are each measured, and these signals are processed and combined to define the tangent of the phase shift caused by rotation of the loop. The rotation rate may then be calculated from this information. Because of the limited range of presently available analog to digital converters which are used with this device, the device cannot provide the necessary dynamic range at the resolution which is required in many gyroscope applications, such as many types of navigation.
In order to overcome some of the problems associated with the techniques described above, various other closed loop approaches have been developed. For example, several closed loop techniques include phase modulation of the counter-propagating light waves at a selected frequency. The optical output signal produced by the light waves is monitored to detect rotation of the loop. When rotation is detected, a feedback signal is produced which controls the phase modulation signal which is applied to the counter-propagating light waves. In response to the feedback signal, the amplitude of the phase modulation signal is adjusted to null out the component in the optical output signal produced by the loop rotation. Thus, the amplitude of the phase modulation signal comprises a measure of the loop's rotation rate.
These closed loop techniques provide the same sensitivity level which is available in open-loop devices, while also significantly increasing the dynamic range over which the rotation rate may be accurately measured. However, the precision and range of these rotation sensors is limited in application by the capabilities of the various output devices to which the sensors may be connected. For example, the output devices must have a range and resolution which permits measurement of the amplitude of the phase modulation signal for large, as well as very small rotation rates. Output devices to be used with these systems are not presently available with both sensitivity levels and dynamic ranges which approach the requirements for applications such as aircraft navigation. In addition, these systems are inherently more complex than the open-loop systems due to the additional electronic circuitry included therein.
In light of the above, it would be a great improvement in the art to provide an open-loop rotation sensing system and method wherein the rotation rate of an all-fiber-optic gyroscope could be precisely, unambiguously and linearly determined over an extended dynamic range. It would be a further important improvement to provide such a system and method which would utilize presently existing components to produce digital readout of the rotation rate over a substantially unlimited dynamic range.